It is best to have intentional overt spiritual conversations with lost people. Those who respond with curiosity and receptivity should be invited to host a Discovery Group where they will invite some of their family and/or close friends to join them in participating. You may have to take them through 2-3 before they will invite others to participate and that is fine.
Other believers can be invited to participate in a multi-week experience of DBS as part of a training program to get them to begin using the format in their own disciple making efforts. But mixing this type of group with the one mentioned earlier is not preferable. Lost people are often concerned that they know so little of the Bible that their ignorance will show, so they hold back if they are with too many people who know much more than they do.
Inviting one other person who is training to become a Multiplier to sit in on a Discovery Group comprised of a lost family/friendship group can be very valuable. But you need to coach this person before they meet in this setting. They absolutely must not “show off” their extensive Bible knowledge! They need to model a humble learning spirit. They must obey their own “I will…” Statement and attempt to share with the person they name, if they are to return. Their greatest contribution will be serving as a role model of someone on a Discovery journey.
Many Christians will not do what is discussed in the previous paragraph, regretfully. Those who will not, will botch a Discovery Group.